1. Field of the Invention
The invention relates to an ink droplet ejecting method and apparatus of an ink jet printhead.
2. Description of Related Art
According to a known ink jet printer using an ink jet printhead, the volume of an ink flow path is changed by deformation of a piezoelectric ceramic material, and when the flow path volume decreases, the ink present in the ink flow path is ejected as a droplet from a nozzle, while when the flow path volume increases, ink is introduced into the ink flow path from an ink inlet. In this type of printing head, a plurality of ink chambers are formed by partition walls of a piezoelectric ceramic material, and ink supply means, such as ink cartridges, are connected to one end of each ink chamber of the plurality of ink chambers, while at the opposite end of each of the ink chambers is an ink ejecting nozzle (hereinafter referred to simply as "nozzle" or "nozzles"). The partition walls are deformed in accordance with printing data to make the ink chambers smaller in volume, whereby ink droplets are ejected onto a printing medium from the nozzles to print, for example, a character or a figure.
As this type of an ink jet printer, a drop-on-demand type ink jet printer which ejects ink droplets is popular because of a high ejection efficiency and a low running cost. As an example of the drop-on-demand type there is known a shear mode type using a piezoelectric material, as is disclosed in Japanese Published Unexamined Patent Application No. Sho 63-247051.
As shown in FIGS. 12A-13, (which are also applicable to the instant invention), this type of an ink droplet ejecting apparatus 600 comprises a bottom wall 601, a top wall 602 and shear mode actuator walls 603 located therebetween. The actuator walls 603 each comprise a lower wall 607 bonded to the bottom wall 601 and polarized in the direction of arrow 611 and an upper wall 605 formed of a piezoelectric material, the upper wall 605 being bonded to the top wall 602 and polarized in the direction of arrow 609. Adjacent actuator walls 603, in a pair, define an ink chamber 613 therebetween, and next adjacent actuator walls 603, in a pair, define a space 615 which is narrower than the ink chamber 613.
A nozzle plate 617 (FIG. 12B) having nozzles 618 is fixed to one end of the ink chambers 613, while to the opposite end of the ink chambers is connected an ink supply source (not shown). On both side faces of each actuator wall 603 are formed electrodes 619, 621, respectively, as metallized layers. More specifically, the electrode 619 is formed on the actuator wall 603 on the side of the ink chamber 613, while the electrode 621 is formed on the actuator wall 603 on the side of the space 615. The surface of the electrode 619 is covered with an insulating layer 630 for insulation from the ink. The electrode 621 which faces the space 615 is connected to a ground 623, and the electrode 619 provided in each ink chamber 613 is connected to a controller 625 which provides an actuator drive signal to the electrode.
The controller 625 applies a voltage to the electrode 619 in each ink chamber, whereby the associated actuator walls 603 undergo a piezoelectric thickness slip deformation in directions to increase the volume of the ink chamber 613. For example, as shown in FIG. 13, when voltage E(V) is applied to an electrode 619c in an ink chamber 613c, electric fields are generated in the directions of arrows 629, 631 and 630, 632 respectively in actuator walls 603e and 603f, so that the actuator walls 603e and 603f undergo a piezoelectric thickness slip deformation in directions to increase the volume of the ink chamber 613c. At this time, the internal pressure of the ink chamber 613c, including a nozzle 618c and the vicinity thereof, decreases. The applied state of the voltage E(V) is maintained for only a one-way propagation time T of a pressure wave in the ink chamber 613c. During this period, ink is supplied from the ink supply source.
The one-way propagation time T is a time required for the pressure wave in the ink chamber 613 to propagate longitudinally through the ink chamber. Given that the length of the ink chamber 613 is L and the velocity of sound in the ink present in the ink chamber 613 is a, the time T is determined to be T=L/a.
According to the theory of pressure wave propagation, upon the lapse of time T, or an odd-multiple time thereof, after the above application of voltage, the internal pressure of the ink chamber 613c reverses into a positive pressure. In conformity with this timing, the voltage being applied to the electrode 619c in the ink chamber 613c is returned to 0 (V). As a result, the actuator walls 603e and 603f revert to their original state (FIG. 13) before the deformation, whereby a pressure is applied to the ink. At this time, the above positive pressure and the pressure developed by reverting of the actuator walls 603e and 603f to their original state before the deformation are added together to afford a relatively high pressure in the vicinity of the nozzle 618c in the ink chamber 613c, whereby an ink droplet is ejected from the nozzle 618c. An ink supply passage 626 communicating with the ink chamber 613 is formed by members 627, 628.
Heretofore, in this type of an ink droplet ejecting apparatus 600, when jet pulses (an optimum pulse width is an odd-multiple value of T) are applied to an actuator continuously at a predetermined frequency to effect a continuous dot printing and when the continuous dot printing is followed by, for example, a one-dot rest and subsequent input of the next dot printing instruction, the ink droplet speed and the direction of droplet ejection become unstable at the portion of the printing instruction under the influence of remaining meniscus oscillation of the ink present in the nozzle concerned, thus giving rise to the problem that a printing line is curved or thinned at that portion, resulting in deterioration of the print quality.
In the case where an ink droplet of a small volume is to be ejected for enhancing the printing resolution, it has been proposed to add, for one dot, a non-jet pulse after application of a jet pulse and before completion of ink ejection. In this case, the remaining meniscus oscillation is suppressed and the ejection of ink becomes stable in a continuous dot printing, but there arises the problem that the energy efficiency is low because it is necessary to continue adding the non-jet pulse. In both cases noted above, the printing instruction is issued without considering whether there is ejection of ink just before and just after the dot concerned.
Now, with reference to FIGS. 1A, 1B and FIGS. 2 and 3, a description will be given of results obtained by conducting two printing operations and actually measuring ink droplet ejecting speeds. FIG. 1A shows a jet pulse signal A (designated the first driving waveform) of pulse width 1 T for one dot and FIG. 1B shows the jet pulse signal A of pulse width 1 T for one dot and a non-jet additional pulse signal B (both designated the second driving waveform). In this case, a time difference between a fall timing of the jet pulse signal A and a rise timing of the additional pulse signal B is set at 2.25 T and that the pulse width of the additional pulse signal B is set at 0.5 T. Here there was used a certain waveform (the first or the second driving waveform) irrespective of whether there is ejection of ink. Table 1 below shows measurement data on the ink droplet ejecting speed (m/s) obtained by a continuous dot printing (1.about.5) with use of each driving waveform, subsequent one-dot rest (6) and subsequent two-dot printing (7, 8). Printing frequency was set at 10.0 kHz. As is seen from Table 1, the ink droplet ejecting speed greatly decreases at the second dot (8) after the rest which follows the continuous printing using the first driving waveform.
TABLE 1 DRIVING DOT WAVEFORM 1 2 3 4 5 6 7 8 1.sup.ST DRIVING 8.0 9.0 9.5 9.5 9.5 -- 9.2 6.5 WAVEFORM 2.sup.ND DRIVING 8.0 7.5 8.1 8.1 8.1 -- 8.0 7.5 WAVEFORM
In the case where printing is performed at a high frequency in such a manner that a continuous dot printing is followed by a one-dot rest and subsequent printing with plural dots, there arises the problem that the second dot after the rest cannot be ejected or the ink droplet of the second dot becomes smaller in continuous dot printing.